1. Field of the Invention
The present invention relates to a time limit function utilization apparatus, particularly, to a semiconductor integrated circuit formed from an age-based change device (aging device) whose output changes over time and a circuit technology controlling the life time of the aging device, more particularly, to an integrable electronic timer which accurately operates in an off-line state in which the timer is disconnected from a battery.
2. Description of the Related Art
A security system which sets an expiration date on a cipher or password has widely been used. For example, in satellite broadcasting, an expiration date is set on an encryption key, and the user is obligated to change the password at predetermined intervals, enhancing security.
For example, the following technique has been reported (see, e.g., Jpn. Pat. Appln. KOKAI Publication No. 10-189780). A nonvolatile semiconductor memory whose data holding life time is arbitrarily set is used for a memory card, commutation ticket, or the like. Data is held for a predetermined period, and after the lapse of the predetermined period, the data is deleted to inhibit the use of the memory card, commutation ticket, or the like.
The nonvolatile semiconductor memory determines the data holding life time by adjusting the atomic composition ratio in each gate insulating film of the nonvolatile memory which constitutes a memory. It is therefore difficult to reproduce an accurate holding life time. In order to form a plurality of memory areas with an arbitrarily set expiration date, memories having gate insulating films with different atomic composition ratios must be formed on a single substrate, which requires a complicated manufacturing method. Undesirably, the holding time can be easily prolonged by accessing a nonvolatile memory and refreshing data.
A technique capable of, even if power is cut, calculating and automatically setting the current time upon power-on again has also been reported (see, e.g., Jpn. Pat. Appln. KOKAI Publication No. 9-127271).
According to this technique, the lapsed time is measured by using a change in the threshold of a memory device such as an EPROM device. The lapsed time between power-off and the next power-on is calculated from a change in the threshold of the memory device, and added to the power-off time, obtaining the current time.
A time cell technique of determining the lapsed time from the discharge rate at which a charge accumulation element looses electrostatic charges via an insulator has also been reported. The time cell can be so programmed as to select a specific period to be measured (see, e.g., Jpn. Pat. Appln. KOKAI Publication No. 2002-246887).
The latter two techniques measure the lapsed time, detecting the threshold voltage change or the discharge rate change, by monitoring charge leakage from the floating gate of a memory cell. Thus, the two techniques are essentially the same and are said to be an age-bases change device (aging device).
A nonvolatile memory cell can be used as one means for implementing an electronic timer without any battery. An EEPROM with a two-layered gate structure of a floating gate and control gate generally has a charge holding function for almost 10 years. The charge holding period (life time) can be shortened by forming a tunnel oxide film as thin as 7 nm between the substrate and the floating gate. The precise control of the charge holding period can implement a battery-less electronic timer (BLET).
In an EEPROM of this type, if the film thickness of the tunnel oxide film varies in the manufacture, the life time greatly varies. For example, the film thickness for all bits is made to fall within an error of ±5% in a process for a 6-nm film thickness of the tunnel oxide film. At this time, as shown in FIG. 119, the gate leakage current which determines the life time of an aging device becomes 20 times larger for −5%, and becomes as small as 1/20 for +5%. Such great variations in leakage current lead to a large difference in the life time, which cannot be permitted in electronic timers.
This is a serious problem in manufacturing an aging device.
When an electronic timer without any battery is implemented using an aging device whose output changes over time, it is difficult to set an accurate operation time because manufacturing variations (of not only the tunnel oxide thickness but also other cell structure parameters) in aging device influence the life time.
Demands have arisen for the advent of a semiconductor integrated circuit capable of suppressing the influence of the presence of a false bit or manufacturing variations in aging device structure parameters (tunnel insulating film thickness, impurity concentration, junction area, gate end shape, and the like) on the life time of the aging device, imposing a time limit to the memory information, and enhancing the controllability of the electronic timer time.